<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Box-sizing</title>
  <style>
    li {
      width: 20px;
      height: 50px;
      float: left;
      /*box-sizing: border-box;*/
      background-color: pink;
      list-style: none;
      padding: 20px 15px;
      margin: 12px 25px;
      text-align: center;
      line-height: 50px;
    }
  </style>
</head>
<body>
  <div class="class">
    <li class="1">1</li>
    <li class="2">2</li>
    <li class="3">3</li>
  </div>
</body>
</html>